1. Field of the Invention
The present invention relates to a hydraulic control system for a continuously variable V-belt transmission including a hydrodynamic transmission unit.
2. Description of the Prior Art
As a control system for a conventional continuously variable V-belt transmission (hereinafter in the present specification referred to as a "continuously variable transmission"), a hydraulic control system is known from Australian pat. 52,356/79, which is shown in FIG. 1. Referring to FIG. 1, oil within a tank 201 is discharged into an oil conduit 204 by an oil pump 203 after passing through a filter 202 and supplied thereafter to a line pressure regulator valve 205. The line pressure provided after pressure regulation by the line pressure regulator valve 205 is supplied on one hand to a cylinder chamber 206a of a driven pulley 206 and on the other hand to a cylinder chamber 208a of a drive pulley 208 via a shift control valve 207. The shift control valve 207 feeds to the cylinder chamber 208a a predetermined oil pressure in response to balance between a rightward directed force created by a spring 210 resulting from rotatable movement of a throttle cam 209 and a leftward directed force produced by oil pressure from the oil conduit 211, thus controlling a shift between the drive pulley 208 and driven pulley 206. It is constructed and arranged such that the oil pressure responsive to revolution speed of the drive pulley 208 builds up in the oil conduit 211. The oil conduit 211 is connected also to the line pressure regulator valve 205 to create the leftward directed force in the line pressure regulator valve 205. To the line pressure regulator valve 205, an axial movement of the drive pulley 208 is transmitted via a rod 212, a lever 213, a slider 214 and a spring 215 such that a force increasing as the reduction ratio between the pulleys 208 and 206 increases is impressed upon the line pressure regulator valve 205. The line pressure regulator valve 205 is constructed and arranged such that oil pressure is produced which increases as the rightward directed force increases and the leftward directed force decreases, so that the line pressure increases as the reduction ratio increases and the line pressure increases as the revolution speed of the drive pulley 208 decreases. That is, the line pressure is controlled in response to reduction ratio and revolution speed of the drive pulley.
However, since, as mentioned above, in a conventional continuously variable transmission, the line pressure is controlled only by reduction ratio and revolution speed of a drive pulley and has no connection with output torque of an engine, it is necessary to set the line pressure sufficiently high enough not to permit slip between the V-belt and pulleys upon production of a maximum torque by an engine for the purpose of always ensuring a torque transmission capacity by the V-belt necessary from the range where the engine produces a small output to the range where the engine produces a large output. It therefore follows that the line pressure is higher than necessary when the output torque of the engine is low, thus applying an oil pressure higher than necessary to the V-belt, thus creating a problem that the endurance of the V-belt drops and a problem that an oil pump must be used which discharges such an unnecessarily high pressurized oil. Besides, the power transmission efficiency of a V-belt gets worse as the difference between the maximum transmission capacity of the V-belt and the actual transmission drive increases, so that it is not desirable from the standpoint of the efficiency either that the oil pressure is higher than necessary.